1. Field of the Invention
The subject invention relates to heat-curable resin compositions. More particularly, the subject invention relates to bismaleimide resin systems which are cured in the presence of derivatized epoxy resins. The thermosetting resin systems find uses in structural adhesives and as matrix resins for prepregs and advanced structural composites.
2. Description of the Related Art
Both epoxy resins and bismaleimide resins have found increasing usefulness in high strength, high temperature applications in the structural materials field, for example, as candidates for structural adhesives and as matrix resins useful in heat-curable, fiber-reinforced prepregs. The bismaleimide resins, in particular, are noted for their high strength performance at elevated temperatures. Unfortunately, bismaleimide resins tend to be somewhat brittle, and thus subject to impact-induced damage. This susceptibility limits applications of the bismaleimides in the aerospace and transportation fields.
Epoxy resins generally cannot match the high strength characteristics of the bismaleimide resins in high temperature applications. Nevertheless, epoxy resins continue to be the major adhesive and prepregging resins in the structural materials field because of the higher flexibility and impact resistance which epoxies have to offer.
It would be desirable to modify the more brittle bismaleimide resins with epoxy resins. Attempts to do so have, thus far, not proven successful for several reasons. For example, it is known that epoxy resins are incompatible with the bismaleimide monomers and precipitate upon standing, as evidenced by U.S. Pat. Nos. 4,131,632 and 4,212,959. This physical incompatibility of the uncured monomers may result in the preparation of resin films or prepregs which do not have the desired monomer contents due to the settling out of the monomer precipitate. It may also cause local non-homogeneity which may unexpectedly alter the physical properties of the cured resin. In addition to these problems of physical imcompatibility, however, the epoxy and bismaleimide resins are chemically incompatible in the sense that they do not cross-cure.
In addition to the incompatibilities discussed above, the curing agents generally used with epoxy resins can create additional problems. Epoxy resins in high temperature, high strength applications are generally cured with amine-type curing agents, particularly 4,4'-diaminodiphenylsulfone and 4,4'-diaminodiphenylmethane. Unfortunately, these amine curing agents are also reactive with maleimides through a Michael-type reaction across the double bond. This reaction has been used in the past to prepare polyaminobismaleimides.
The reactivity of these amines with both epoxies and bismaleimides has the effect of altering the epoxy-amine stoichiometry. Moreover, it is difficult to overcome this effect because the magnitude of the various reactions is affected by the type of catalyst present, by the storage conditions of the uncured resin, and by the cure and postcure conditions.
It would be desirable to be able to formulate a resin system which can take advantage of the properties of both epoxy and bismaleimide resins without the above-mentioned problems. Such a resin system should provide a storage-stable, homogenous system which can cure in a predictable manner.